How many grams of dry glucose do you need to add to 400 g of 5% glucose solution to make an 8% solution?

Given:
m1 (solution) = 400 g
W1 = 5%
W2 = 8%
To find:
m2 (substances) =?
Decision:
1) Let’s write down the formulas for solving the problem.
W = (m substance: m solution) x 100%
m substance = (W x m solution): 100%
2) Find the mass of dry glucose for the first solution.
m substance = (5% x 400 g): 100% = 20 g
3) Find the mass of dry glucose for the second solution.
W2 (8%) = ((20 + x): (400 + x)) x 100%
0.08 = ((20 + x): (400 + x))
0.08 x (400 + x) = 20 + x
32 + 0.8x = 20 + x
12 = 0.92x
x = 13.
Answer: m2 (substances) = 13 g.